The invention relates to a weighing pan that is particularly suitable for use in a kind of balances where, for example, special requirements are observed in placing a weighing object on the pan, namely in comparator balances, where the weighing pan is designed in a way that serves to protect the weighing object.
User instructions are normally provided to ensure that a balance is used correctly. The handling of the weighing objects when they are placed on the weighing pan and the operations involved in performing the weighing process, particularly in a laboratory environment, are defined by rules and so-called Standard Operating Procedures (SOP). The purpose of such instructions is to prevent damage to the balance as well as the weighing object as much as possible. The more sensitive the balance, the higher the probability that even with utmost diligence the weighing pan as well as the highly sensitive weighing objects will over time show signs of wear. In the following, this problem will be discussed for the case of a comparator balance.
A comparator balance of the kind that is described, e.g., in DE 295 17 368 U1 has to meet the most exacting requirements in regard to resolution and repeatability. The weights whose mass or volume is tested on this type of balance through a comparison with a known reference are assigned to mass-tolerance classes in accordance with international standards. They are further subject to requirements with regard to physical properties such as shape, material, density, surface finish, etc. The primary users of comparator balances are national mass laboratories, governmental Weights & Measures offices as well as independent and company-internal calibration laboratories. A comparator balance, in particular the weighing pan of a comparator balance, should therefore be designed to handle the weights under test as well as the reference weights gently, so that there is no noticeable loss of mass from abrasion even after a million weighing cycles. A loss of mass would manifest itself for example through fine scratches in the surface that is otherwise polished to a mirror finish.
Until now, the occurrence of scratches was tolerated, because the weighing pans consist of metal (aluminum or steel) and are preferably hardened to increase their own wear resistance. As a result, a test weight may have to be reclassified after an extended period of use after a loss of mass has been found.
As a possible solution to this problem, the surface of the test weights can be hardened in accordance with the concept described in U.S. Pat. No. 6,552,280 B1.
In other commercially available comparator balances, the protection of the test weights is achieved by overlaying the weighing pan with a glued-on slab of cork. However, using cork as a damping material on the weighing pan can have a disadvantage that it is not abrasion-resistant and, therefore, small particles may stick to the test weight and subsequently introduce an error in the weighing result. Furthermore, cork ages fast, so that the protective cork overlay will have to be replaced at relatively frequent intervals. Being an organic material, cork is subject to strong variations in its physical properties such as granularity which is associated with surface roughness, or water retention. As a consequence, certain steps in the weighing process, such as the centering of the test weights on a pendulous weighing pan—as described for example in DE 295 17 368 U1—can require frequent readjustments.
In the centering method according to the aforementioned patent document, the test weight initially lies or—for example in the case of OIML weights—stands on a platform. The weighing pan, preferably a suspended type of pan, is configured so that it reaches through the platform which is movable up and down. When the platform with the test weight is lowered, the test weight is transferred from the platform to the weighing pan. If the test weight is not centered on the weighing pan, the latter will seek a new position to align the combined center of gravity of the weight and pan vertically below the suspension point, whereby the weight is moved closer to the center of the platform. An iterative process of lowering and lifting the platform can thus result in a centered position of the weight on the weighing pan. The process of centering the test weights on the weighing pan of a comparator balance can be of critical importance in a precise mass measurement, but the aforementioned procedure is not always successful. In particular, the procedure can fail if, for example, the test weight slides on the weighing pan.
While the friction between the test weight and the weighing pan could be increased by a rougher surface finish of the weighing pan, a weighing pan with an increased surface roughness can cause more mechanical abrasion on the test weight.